For several years now, motor vehicles have been made with a sloping front, thereby making it necessary to slope each headlight glass substantially relative to the vertical plane in which such glasses used to lie, with the angle relative to the vertical sometimes being as much as 70.degree., for example.
Such a sloping glass with conventional, generally cylindrical stripes for spreading light by refraction leads to a well-known phenomenon whereby the more a light ray is deflected sideways, the more it is also deflected downwards. This gives rise to a beam which is generally crescent-shaped, as illustrated for example, in the Applicant's French patent published under the number 2 428 204.
That patent also proposes frustoconical stripes for the purpose of deliberately imparting a well-controlled downwards deflection to the sides of the beam, in particular for improving illumination of the roadside. As a result, that patent neither teaches nor suggests any kind of stripe which, when provided on a sloping glass, is capable of deflecting a horizontal light ray without imparting any downwards deflection thereto.
Another French patent, also in the name of the present Applicant and published under the number 2 542 422, describes a stripe for a sloping glass which is capable of imparting sideways deflection to an incident ray without imparting any downwards deflection thereto. However, the stripes taught by that patent are capable of achieving this result only under the severe limitation whereby the angular range of deflections imparted to rays by each stripe is extremely small. As a result it is necessary to provide a very large number of stripe zones, which each zone providing a given degree of deflection and including stripes having a shape specific to the zone, thereby ensuring that the various relatively narrow light spots corresponding to each zone of stripes are suitably complementary to one another. This makes construction of the glass relatively complicated and may give rise to a final beam which is lacking in uniformity.
The present invention seeks to mitigate these drawbacks of the prior art and to propose a stripe which, designed for a glass of arbitrary slope, is capable on its own of spreading its portion of the incident beam very widely and uniformly without imparting any dowwards deflection to the light rays relative to the essentially horizontal plane in which they are originally contained. More precisely, the invention seeks to propose a stripe which, on receiving a concentrated beam of rays parallel to the optical axis, is capable of forming a strip of light which is thoroughly horizontal and very uniform.